Industries worldwide have increased significantly in number and have improved their processes over the years. Many of these industries require systems that provide cooling for at least some of their processes. Many of the cooling systems utilize water as a heat sink or heat transfer fluid. However, water is a limited resource. Exploitation and contamination of underground aquifers, oceans, and surface waters has occurred, leading to a decrease in the quantity of suitable water, as well as the quality of the naturally available water. Thus, new ways of using water in a sustainable and economical way need to be found in order to utilize this resource in an efficient manner and without damaging the environment.
Current industrial cooling systems are often restricted to areas where large volumes of cooling water are available. For example, cooling systems are often located along an ocean coast line or near other natural sources of water, such as rivers and large lakes, where this resource exists abundantly. Accordingly, a significant drawback associated with water-based cooling systems is that they are often constrained to specific geographical areas. For example, for a 350 MW power plant using coal, nearly 45,000 cubic meters of water per hour may be needed for cooling purposes, such as in plant heat exchangers, equivalent to filling 18 Olympic swimming pools in just one hour.
Moreover, waste heat absorbed by the cooling water generally is lost into the environment by discharging the heated water back into a natural source of water, or by discharging water vapor into the atmosphere. Recoverable energy that is wasted throughout the world each day may be up to 80% of the total electricity consumed daily worldwide.
Specific representative environments which may benefit from improved industrial water-based cooling systems can include, but are not limited to, the following:
Thermal Power Plants
Increases in population and technological advances have resulted in a vast demand for additional energy. A significant use of worldwide energy is concentrated in the generation of electricity. The demand for electricity is growing at a pace set by the modernization of nations and their economic development. For example, electricity generation has increased nearly 40% in the last 10 years (see FIG. 1). This demand has led to an increase in the construction of new facilities for electricity generation worldwide.
Thermal power plants are currently the predominant type of power plants in operation. These plants employ a fuel to generate combustion, with the combustion heating a fluid which in turn drives a turbine in an electrical generation circuit. There are also a number of power plants using renewable resources—such as solar energy or geothermal energy—generating a driving fluid that in turn drives a turbine. Still other thermal power plants use nuclear fuels, such as uranium. However, available statistics show that of the total energy consumed in 2008, 80% to 90% was derived from the combustion of fossil fuels in thermal power plants. Most typically, these types of plants use coal, oil, or natural gas. In part, this large percentage of electricity production is due to the high availability of fossil fuels in the world. In 1973, the world energy matrix consisted of 78.4% thermal power plants (including nuclear plants), while in 2008 the percentage had increased to 81.5%. There is a continual need for these plants to improve their operational efficiency and reduce their environmental impact.
Over time, thermal power plants have undergone diverse changes associated with their operation. For example, changes have been implemented relating to emissions and the efficient use of fuel. However, a remaining drawback of these plants is the use of water cooling systems. These systems have several disadvantages which restrict the use to certain geographical locations. Additionally, the use of the water and attendant heating of the water produce a potentially damaging impact to the environment, raise energy costs, result in an intensive use of water, waste the residual heat, and/or have high installation and operation costs. Accordingly, improved cooling systems are needed to keep up with the growing demand for energy and electricity.
The current cooling systems used in thermal power plants and other industries are: once through cooling systems, wet cooling towers, and cooling ponds.
Once Through Cooling Systems
One of the main types of cooling systems used today is the “once through” cooling system, which refers to an open-cycle system (i.e., not employing water recirculation). This type of system consists of a water intake structure to collect the water from a natural source and a discharge structure to return the water back to the natural source (e.g., often the ocean or sea). Collected cooling water is circulated through heat exchangers functioning as part of the industrial process. In heat exchangers, the water acts as a heat sink whereby the water temperature increases as it flows through the exchanger. The heated water is then discharged back into the natural source. In the U.S. alone, approximately 5,500 power plants use a once through cooling system. These plants use more than 180,000 million gallons of water per day for cooling purposes. This amount is, for example, more than 13 times the irrigation water used in Australia daily. Once through cooling systems have many drawbacks including environmental damage due to suction and death of marine organisms; thermal pollution from the returned, heated water; restricted location of the plants to a coastline (or on the border of large water sources); poor quality water; and waste of residual heat.
The once through cooling system uses large volumes of water at relatively low cost, but often leads to large-scale adverse effects on the marine ecosystem. For example, this system creates a temperature increase in the discharged water. In the ocean, the sharp increase in temperature can cause serious problems, even resulting in death of living organisms. This affects the marine ecosystem and human activity that takes place on the coast, such as fisheries and other economic activity. The once through cooling system can also cause the death of marine organisms due to suction produced in the water inlet. This may affect millions of fish, larvae and other aquatic organisms each year worldwide because they are sucked into the conduits leading to the heat exchangers. Death can occur because of the filters or screens (e.g., collisions with filters/screens or retention by the filters or screens), because of the driving pumps (e.g., by passing inside structures at high pressures and/or flows that cause collisions with the walls), due to chemicals that may be added, and in the heat exchangers due to the abrupt change of temperature. The laws of some countries and states ban the use of once through cooling systems. Therefore, there is a need to seek new ways of cooling that are sustainable over time and allow better performance and efficiency.
Another major limitation of the once through cooling system is its restricted location. As noted above, these types of plants must typically be located on the coast bordering the sea or inland along rivers, in order to better capture large amounts of water. These locations can create significant land use problems. These industries are thereby limited due to the large volumes of water to be captured and the effect of thermal pollution in such places. Because of this, plants have various problems related to location which results in higher costs and potential rejection by the residents of the community.
Another problem with the once through cooling system is the poor quality of water used for cooling. Once through cooling systems typically use seawater, which has a large organic content. This adversely affects the heat transfer systems of cooling processes. For example, reduced heat transfer occurs due to living or dead organisms which adhere to or clog the pipes. Biofouling is produced and begins to adhere to the inner surface of the pipes, reducing heat transfer and thus generating greater inefficiencies. In addition, new environmental standards recommend (and some require) that plants operate at a high efficiency to maximize the amount of energy produced per unit of fuel. One study estimates that fouling in heat exchangers produces monetary losses in industrialized countries at a level of about 0.25% of Gross Domestic Product (GDP).
Another constraint of once through cooling systems is that all the heat absorbed is discharged back into the natural water source without using the thermal energy in the water. In some instances, the thermal energy that is wasted can approach two thirds of the total generated heat, while the amount of electric energy produced by a power plant is only one third of the total generated heat. It would be advantageous to use this wasted, valuable energy for other beneficial purposes.
Wet Cooling Towers
Another cooling system currently used is a wet cooling tower. These systems cool water through heat exchange with air inside evaporation chimneys. The chimneys contain a cold water reservoir at the base which feeds the plant by pumps that circulate through the condenser of the plant (chillers), thereby transferring the heat of the working fluid of the plant into the water. When the high temperature effluent water reaches the top of the tower, it begins to descend in fine jets to maximize contact area for heat transfer. Some plants have fans, either on the top or bottom of the tower, to circulate air upwards so as to achieve a counter-flow contact with water. As the water falls, it cools and heat loss occurs through evaporation. When water evaporates, dissolved salts fall back into the water tank, thereby increasing its concentration. Therefore, a certain amount of water must be purged from time-to-time and the reservoir must be fed with fresh water. Wet cooling towers have various problems associated with their operation, including high withdrawal rates and evaporation of water, high costs, deterioration of the urban aesthetic or landscape, and loss of the captured residual heat.
A significant problem of wet cooling towers is the high rate of water use. According to the Electric Power Research Institute (EPRI), for a steam driven power station operating on coal, water withdrawal rates are about 2,082 liters/MWh, and water consumption due to evaporation is about 1,817 liters/MWh. Moreover, wet cooling towers require frequent replenishment due to heavy water consumption caused by high evaporation rates. All the evaporated water must be replaced and also from time to time a certain amount of water must be purged due to the increase in mineral concentration in the tank, which also must be replenished. Generally, wet cooling towers work with fresh water, resulting in higher operating costs.
Another major problem of wet cooling towers is that they have high installation, operation, and maintenance costs. For example, for a plant of 2,245 MW, the capital cost may rise to 600 million dollars.
Further, wet cooling towers cause a deterioration of urban aesthetics and landscape. This is due to both the structure of the tower and the steam discharged from the tower into the atmosphere. The steam interferes with the landscape view and can cause wet pavements, roads, and other adjacent surfaces. A further limitation of wet cooling towers is that they do not exploit the residual energy, since they discharge virtually all the residual heat into the atmosphere as water vapor. Accordingly, the overall energy efficiency of the process is reduced.
Cooling Ponds
Many current cooling systems used in industrial processes employ cooling ponds. Cooling ponds generally consist of large volumes of water contained in a pond from which cooling water is extracted. After going through a heat exchange process in the plant, the water (with a higher temperature) is discharged back into the pond. The area of the pond typically depends on the capacity and efficiency of the plant. These types of ponds are used by almost fifteen percent (15%) of thermal power generation plants in the U.S. that use coal, other fossil fuels, a combined cycle, and nuclear plants. The main disadvantages of cooling ponds are the large physical areas required for implementation and the poor quality of water contained within the pond.
The requirement of a large area for cooling pond implementation is based on the low temperatures to be maintained—generally below 22° C. This is because once the water temperature begins to rise, the pond water is more prone to the growth and proliferation of algae and other organisms that cause problems in the cooling system and the pond itself. So to maintain low temperatures, cooling ponds have very large areas of up to 2,500 hectares. Considering that land use is increasingly scarce, this solution is becoming less viable.
Another limitation of cooling ponds is the poor quality of water in the pond. In some plants, the cooling water from the pond must be subjected to additional treatments such as filtration and removal of compounds that damage machinery. The poor quality is due to the proliferation of microorganisms, algae, and sediment particles. Water quality in these ponds makes them unattractive for use in recreational purposes, and they may pose health hazards to people who use the pond.
Also, since the water temperature in the cooling pond is not permitted to increase to 25-30° C. or more, the heated water cannot be used for other purposes, therefore wasting valuable thermal energy.
Casting Industries
Other industries, such as foundry and cast industries, may use a cooling water system. The foundry industry is of high importance, especially for mining where metals are melted to produce other products. In the casting process, gases are generated at extremely high temperatures, which must be cooled for later discharge or use. Currently, most foundry industries use water cooling systems, either by recycling or by once through cooling systems.
Based on the cooling needs of many industries and the drawbacks of existing cooling systems, there is a need for improved cooling systems which operate at a lower cost, avoid thermal pollution and associated thermal damage to marine ecosystems, use less water, allow for flexibility in geographic locations, and/or take advantage of the thermal energy generated by the cooling process (e.g. heat exchanger) for useful purposes.